2,3,5,6-Tetrachloropyridine and 2,3,5-trichloropyridine are chemical intermediates which are extensively used in the production of commercial insecticides and herbicides. Their production has been the object of many investigations and new methods having improved economics or other desirable features continue to be sought. Methods which employ available starting materials of much lower value or which might otherwise be disposed of as waste are particularly valuable. The present invention relates to such a method.
The vapor phase chlorination of pyridine to obtain various mono- and polychloropyridine products is known. In one study, the intermediate mono- and polychloropyridine compounds were identified and the rate constants for each chlorination event at 500.degree. C. were determined (H. Suschitzky, Polychloroaromatic Compounds (1974), page 440). Analysis of the rate constants given leads to the conclusion that under these conditions 2,3,6-trichloropyridine will produce 2,3,5,6- and 2,3,4,6-tetrachloropyridines in an approximately 3 to 1 ratio and 2,6-dichloropyridine will produce 2,3,6- and 2,4,6-trichloropyridines in an approximately 1 to 3 ratio.